What is in this article?:
- Fluid Power eBook â Fluid Power Circuits Explained
- Chapter 1: Hydraulic Accumulators
- Chapter 2: Air Logic Circuits
- Chapter 3: Air-Oil Circuits
- Chapter 4: Slip-In Cartridges
- Chapter 5: Counterbalance Valve Circuits
- Chapter 6: Fluid Power Cylinders
- Chapter 8: Directional Control Valves
- Chapter 9: Filtration
- Chapter 10: Flow Control Circuits
- Chapter 11: Flow Divider Circuits
- Chapter 12: Fluid Motor Circuits
- Chapter 13: Pressure Intensifier Circuits
- Chapter 14: Proportional Control Valve Circuits
- Chapter 15: Pumps
- Chapter 16: Reducing Valves
- Chapter 17: Regeneration Circuits
- Chapter 18: Pressure-relief Valves
- Chapter 19: Rotary Actuator
- Chapter 20: Sequence Valve Circuits
- Chapter 21: Servovalve Circuits
- Chapter 22: Synchronizing Circuits
- Chapter 23: Sample Actual Circuits
Chapter 8: Directional Control Valves
Directional Control Valves
Directional control valves perform only three functions:
- stop fluid flow
- allow fluid flow, and
- change direction of fluid flow.
These three functions usually operate in combination.
The simplest directional control valve is the 2-way valve. A 2-way valve stops flow or allows flow. A water faucet is a good example of a 2-way valve. A water faucet allows flow or stops flow by manual control.
A single-acting cylinder needs supply to and exhaust from its port to operate. This requires a 3-way valve. A 3-way valve allows fluid flow to an actuator in one position and exhausts the fluid from it in the other position. Some 3-way valves have a third position that blocks flow at all ports.
A double-acting actuator requires a 4-way valve. A 4-way valve pressurizes and exhausts two ports interdependently. A 3-position, 4-way valve stops an actuator or allows it to float. The 4-way function is a common type of directional control valve for both air and hydraulic circuits. A 3-position, 4-way valve is more common in hydraulic circuits.
The 5-way valve is found most frequently in air circuits. A 5-way valve performs the same function as a 4-way valve. The only difference is an extra tank or exhaust port. (Some suppliers call their 5-way valves, “5-ported 4-ways.") All spool valves are five ported, but hydraulic valves have internally connected exhaust ports going to a common outlet. Because oil must return to tank, it is convenient to connect the dual tank ports to a single return port. For air valves, atmosphere is the tank, so exhaust piping is usually unimportant. Using two exhaust ports makes the valve smaller and less expensive. As will be explained later, dual exhausts used for speed-control mufflers or as dual-pressure inlets make this configuration versatile.
Following are schematic symbols for commonly used directional control valves.
2-way directional control valves
A 2-way directional valve has two ports normally called inlet and outlet. When the inlet is blocked in the at-rest condition, as shown in Figure 8-1, it is referred to as "normally closed" (NC). The at-rest box or the normal condition is the one with the flow lines going to and from it.
The boxes or enclosures represent the valve’s positions. In Figure 8-1, the active box shows blocked ports, or a closed condition, while the upper box shows a flow path. When an operator shifts the valve, it is the same as sliding the upper box down to take the place of the lower box. In the shifted condition there is flow from inlet to outlet. Releasing the palm button in Figure 8-1 allows the valve spring to return to the normal stop flow condition. A 2-way valve makes a blow-off device or runs a fluid motor in one direction. By itself, a 2-way valve cannot cycle even a single acting cylinder.
Figure 8-2 shows a "normally open" (NO) 2-way directional valve. Energizing the solenoid on this valve stops fluid flow.
Valve operators come in different types. Figure 8-3 shows a solenoid pilot operator using solenoid-controlled pressure from the inlet port to move the working directional spool. Figure 8-4 shows a cam-operated valve. A moving machine member usually operates this type valve.
3-way directional control valves
A 3-way valve has three working ports. These ports are: inlet, outlet, and exhaust (or tank). A 3-way valve not only supplies fluid to an actuator, but allows fluid to return from it as well. Figures 8-5 through 8-10 show schematic symbols for 3-way directional control valves.
Figure 8-6 depicts an all-ports-blocked, 3-way, 3-position valve. A valve of this type connected to a single-acting, weight- or spring-returned cylinder could extend, retract, or stop at any place in the stroke.
Some 3-way valves select fluid flow paths as in Figure 8-9. Use a spool-type valve for this operation. Another flow condition is the diverter valve shown in Figure 8-10. A diverter valve sends fluid to either of two paths.
Figure 8-9. Solenoid pilot-operated 3-way selector valve.
| Figure 8-10. Palm-button-operated 3-way diverter valve. |
4-way directional control valves
Figures 8-11 to 8-15 show different configurations available in 4-way directional control valves. They range from the simple, two-position, single, direct solenoid, spring-return valve shown in Figure 8-11, to the more complex three-position, double solenoid, pilot-operated, spring-centered, external-pilot supply, external drain valve shown in Figure 8-15.
| Figure 8-11. 4-way, 2-position direct solenoid-operated spring return. |
Lines to the boxes show flow to and from the valve, while lines with arrows in the boxes show direction of flow. The number of boxes tells how many positions the valve has.
INSERT FIG 8-12
| Figure 8-12. 4-way, 2-position direct solenoid-operated spring centered. |
Figure 8-12 shows a single solenoid, spring-centered valve. This valve has a third position but there is no operator for it. Use this spring-centered, single solenoid valve in control circuits for special functions. In the past, to get this configuration, you only had to wire one solenoid of a double-solenoid, three-position valve.
Figure 8-13. 4-way, 2-position direct solenoid-operated spring return.
Figure 8-13 shows another unusual 4-way configuration. This valve shifts from an actuator moving flow path to center condition for certain special circuits.
5-way directional control valves
Figures 8-16 through 8-20 show symbols of some 5-way air valves. Most spool-type air valves come in a 5-way configuration. Because air usually exhausts to atmosphere, the extra exhaust port is no problem.
Figure 8-14. 4-way, 2-position solenoid, pilot operated detented, line-mounted.
|Figure 8-15. 4-way, 3-position solenoid, pilot operated centered, manifold-mounted.|
| Figure 8-16. Line-mounted, air-pilot-operated, 2-position, detented 5-way valve. |
Many valves use the two exhaust ports for speed control mufflers. Mufflers not only make the exhaust quieter, but throttle the exhaust, which in turn controls cylinder speed in a meter-out circuit.
Another example later in this section shows dual exhaust ports piped with different pressures to save air. Also use dual inlet piping to make an air cylinder operate quickly and smoothly. (See Figures 8-48 through 8-55.)
Figure 8-17.Line-mounted, solenoid pilot-operated, 2-position,spring returned 5-way valve.
| Figure 8-18. Line-mounted, hand lever-operated, 2-position, spring-returned 5-way valve. |
Most air cylinders stroke from one extreme to the other. A two position, single solenoid, spring return valve is sufficient for this operation. About 90% of air circuits use this type of valve. To stop an air cylinder in mid-stroke, use the 3-position valve shown in Figures 8-19 through 8-21.
It is difficult — if not impossible — to accurately stop an air cylinder any place other than at end of the stroke. When the cylinder moves slowly, a repeatable mid stroke position of plus or minus an inch might be possible. The problem is, if the load on the cylinder changes or there is any slight leak in the piping or seals, it will not hold position once it stops.
|Figure 8-19. 5-way, 3 position, spring-centered solenoid, pilot-operated, cylinder ports open-center condition, line mounted.|
Figure 8-20. 5-way, 3 position, spring-centered solenoid, pilot-operated, all ports blocked center condition, line mounted.
| Figure 8-21. 5-way, 3 position, spring-centered pressure to cylinder ports, exhausts blocked center condition, solenoid-pilot operated, line mounted. |
Three-position valves come in several styles, including: cylinder ports open as seen in Figure 8-19; all ports blocked as seen in Figure 8-20; and pressure to cylinder ports as seen in Figure 8-21.
Using 2-way valves
Figures 8-22, 8-23, and 8-24 show some uses for 2-way directional control valves.
One use is the blow-off function shown in Figure 8-22. A 2-way valve in Figure 8-23 operates a one-direction motor with an open exhaust in the motor housing. The circuit in Figure 8-24 works well for electrically unloading a pump for easy start up and/or reduced heat generation
Figure 8-22. Blow-off.
Figure 8-23. Running a one-direction fluid motor.
Figure 8-24. Unloading a pump.
Figure 8-25 shows a weight-returned, single-acting cylinder powered by a 2-way in the <b>at rest</b> condition. At first sight it looks as if this circuit might work. Shifting the 2-way valve, or extending, sends fluid to the cylinder cap end and it extends. The problem comes when the 2-way returns to normal at the end of cycle. Instead of the cylinder retracting after the solenoid de-energizes, it stays in the extended position. The cylinder would only return if the valve, cylinder seals, or pipe connections leak.
Figure 8-25. Using one 2-way valve to operate a single-acting cylinder.
Figure 8-26 shows a circuit that operates a single-acting cylinder with 2-way valves. One (NO) and one (NC) 2-way directional valve piped to the cap end cylinder port allows fluid to enter and exhaust from it. Actuating both operators simultaneously extends the cylinder. According to valve size and inlet air flow, the cylinder might not extend if just energizing the (NC) valve. If the cylinder extends with only one valve actuated, it would be slow and waste a lot of air.
Figure 8-26. Operating a single-acting cylinder with two 2-way valves.
Figure 8-27 shows four 2-way valves piped to operate a double-acting cylinder. A pair of 2-way valves at each cylinder port gives a power stroke in both directions. Energize and de-energize all four valves simultaneously to cycle the cylinder and keep from wasting fluid.
Figure 8-27. Operating a single-acting cylinder with four 2-way valves.
Four 2-way valves may seem to be a complex and expensive way to operate a cylinder. However, in the past few years, poppet type slip-in cartridge valves have been operating large bore hydraulic cylinders this way. See chapter four on Cartridge Valves for the advantages of these valves in high flow circuits.
Using 3-way valves
Figure 8-28 shows a 3-way valve, used to select Pr. 1 or Pr. 2. Use a spool type directional control valve in this type of circuit. Spool valves normally take pressure at any port without malfunction. Poppet design valves normally take pressure at the inlet port only.
Figure 8-28. Pressure selector.
Since the example selector valve is solenoid pilot-operated, it is important to determine which port has the higher pressure. Most solenoid pilot-operated valves take air from the normal inlet port to operate the pilot section. If both inlet pressures are too low to operate the valve, plumb an external pilot supply from the main air system.
When it is necessary to lock out one of two circuits while the other one operates, the hookup in Figure 8-29 works well. While circuit one has fluid going to it, working on circuit two is no problem. Use a spool type valve here also. Poppet valves usually only take pressure at one port.
Figure 8-29. Fluid diverter.
The most common limit valve is a miniature 3-way like the one shown in Figure 8-30. This particular example is (NC). Contact with a machine member opens it. Except for bleeder type control circuits, a limit valve requires at least a 3-way function. Once this normally closed valve shifts, it passes a signal on to continue the cycle. In normal condition, fluid in the control circuit exhausts through the exhaust port.
Figure 8-30. NC limit valve.
Figure 8-31 shows a single-acting cylinder with a 3-way valve powering it. Energizing the solenoid, or extending, allows flow to move to the cylinder port and it extends. Deenergizing the solenoid or retracting, lets the valve shift to home position, and the cylinder retracts from outside forces. The exhaust port on a 3-way valve lets fluid in the cylinder escape to atmosphere.
Figure 8-31. Operating a single-acting cylinder with one 3-way valve.
To operate a double-acting cylinder with 3-way valves, use the hookup shown in Figure 8-32. With a 3-way directional valve at both ports, both extend and retract strokes of a double-acting cylinder have force.
Figure 8-32. Operating a double-acting cylinder with two 3-way valves.
Some manufacturers use dual 3-way valves to conserve air. Piping between the valve and cylinder ports wastes air. Every time a cylinder cycles, the lines to both ports fill and exhaust. The longer the valve-to-cylinder lines are, the greater the air waste. Mounting air valves directly to the cylinder ports minimizes air waste. The higher cycle rate results in greater savings.
Lowering pressure at the rod end port is another way to save air with dual 3-way valves mounted directly to the cylinder port. As discussed before, reducing air pressure at the cylinder uses less compressor horsepower. Usually, force required to return a cylinder is minimal, so lower pressure at the rod port saves energy.
Speed-control mufflers in the direct-mounted 3-way valves independently control the extend and retract speed of the cylinder. This saves piping time and the cost of flow control valves.
Figure 8-33 shows an air cylinder inching circuit. It is possible to inch an air circuit if accuracy and repeatability are not important. An inching circuit’s repeatability is usually not closer than ±1 in. if travel speed is slow. Faster travel speeds give less control.
Figure 8-33. Inching circuit for a double-acting cylinder with two 3-way spring-centered valves.
A 3-way valve can replace a 2-way valve. To duplicate the 2-way function, block the exhaust port of the 3-way valve. Blocking the exhaust of a 3-way is usually not necessary for most 2-way applications. Using 3-way valves in place of 2-way valves reduces inventory cost and saves time.
Using 4-way valves
See Figures 8-34 to 8-36 for some uncommon uses of 4-way directional control valves. Using directional controls in ways other than normal is a common practice. Make sure the valve is capable of pressure in all ports before applying it to some of these circuits. If the valve is solenoid pilot-operated, where does pilot supply come from? Also check with the manufacturer if there is any doubt about the valve’s performance in an unusual application.
Figure 8-34. Double flow capacity.
To make a high flow 2-way valve from a 4-way valve try the circuit shown in Figure 8-34. Connect pump flow to the normal inlet port and its outlet port, then connect the other outlet port to the normal tank port and on to the system. In the at-rest condition there is no flow through the valve. When the valve shifts, flow is fromP through B to system and from A through T to system. A valve rated at 10 gpm is now good for 20 gpm with little or no increase in pressure drop. Make sure the valve is capable of backpressure at the tank port.
This piping arrangement comes in handy in hydraulic circuits, since most manufacturers do not offer a 2-way valve. Also, a lot of 2-way hydraulic valves only stop flow in one direction, so they are useless in a bi-directional flow line.
For a full time regeneration circuit, pipe the 4-way as shown in Figure 8-35. Read Chapter 17 for a full explanation of this regeneration circuit.
Figure 8-35. Full time regeneration.
Figure 8-36 shows how to pressurize both ends of the cylinder when a 4-way valve centers. When a cylinder retracts to pick up another part, it often has to go too far to make sure it is behind the part. Low backpressure from the check valve makes the cylinder creep forward at low power so the cylinder is in contact with a part before the next cycle starts.
Figure 8-36. Low-pressure cylinder extend.
Figure 8-37 shows the normal hookup of a 4-way directional valve. A double-acting cylinder only needs one 4-way directional valve to extend and retract it. The three sequences show a 4-way valve in action. Add flow controls or a counterbalance valve to complete the circuit when there is weight on the rod. Note the port hookup is A to cap and B to rod. Using this port connection arrangement consistently makes it is easy to wire the circuit because the electrician knows A solenoid extends the cylinder while B solenoid retracts it. Maintenance persons always know which manual override to push during trouble shooting or setup.
Figure 8-37. Operating a double-acting cylinder with one 4-way valve.
Most hydraulic directional control valves are 3-position. Valve center conditions perform different functions in relation to the actuator and pump.
An all-ports open center condition directional valve unloads the pump and allows the actuator to float as shown in Figure 8-38. This reduces heat build up and allows opposing forces to move the cylinder without building backpressure.
Figure 8-38. Inching circuit with pump unloaded, cylinder floating.
To block the cylinder while unloading the pump, use the center condition shown in Figure 8-39. Most hydraulic valves are a metal-to-metal fit spool design, so do not depend on the cylinder setting dead still with a tandem center spool. If there are outside forces on the cylinder, it will creep when the valve centers.
Figure 8-39. Inching circuit with pump unloaded, cylinder blocked.
If the cylinder needs to float while blocking pump flow, use the center condition shown in Figure 8-40.
Figure 8-40. Inching circuit with pump blocked, cylinder floating.
Figures 8-41 to 8-46 show several commonly used 4-way hydraulic valve center conditions. The first four account for about 90% of all 3-position hydraulic valves in use.
The center condition of a 3-position valve can unload a pump, open actuator ports to tank for free movement, block actuator ports to stop movement, give regeneration, or work in combinations of these functions.
Figure 8-41. All ports open, center condition.
Figure 8-41 shows an all-ports-open center condition valve. The open center condition unloads the pump and allows the actuator to coast to a stop or float. In the crossover or transition condition it causes very little shock. Fixed volume pumps use this center condition.
The all-ports-blocked center condition valve of Figure 8-42 appears to block the cylinder ports. In actual use, leakage oil across the spool lands pressurizes A and B ports, possibly causing a single rod cylinder to extend. This is not a good choice for stopping and holding a cylinder as the symbol seems to indicate. To positively stop a cylinder, use a valve with the cylinder ports hooked to tank, and pilot-operated check valves in the cylinder line or lines. (See the section on “Check Valves as Directional Valves.”)
Figure 8-42. Ports blocked, center condition.
The float center valve of Figure 8-43 allows the actuator to float while blocking pump flow. Pump output is available for other valves and actuators with this center condition. It also works well for pilot-operated check valve locking circuits or with counterbalance valves. This is the normal center condition for the solenoid valve on a solenoid pilot-operated, spring-centered directional valve.
Figure 8-43. Float center condition.
Figure 8-44 shows a tandem center valve. A tandem center valve lets the pump unload while blocking the cylinder ports. The cylinder sits still unless there is an outside force trying to move it. Any metal-to-metal fit spool valve never fully blocks flow. With external forces working on the cylinder, it may slowly creep with the valve centered. This is another common center condition for fixed volume pumps.
Figure 8-44. Tandem center condition.
The regeneration center position of the valve in Figure 8-45 pressurizes and connects both ports of a cylinder to each other. Connecting pressure oil to both cylinder ports and to each other regenerates it forward when the valve centers. This valve is the pilot operator for hydraulically centered directional valves or normally closed slip in cartridge valves.
Figure 8-45. Regeneration center condition.
To unload the pump while blocking the cylinder from moving, use the valve shown in Figure 8-46. However, the metal-to-metal fit spool will not lock the cylinder when there are external forces.
Figure 8-46. Pump unload, port B blocked, center condition.
Figures 8-47 to 8-48 show what is commonly referred to as the “crossover” or “transition” condition of a spool. In some actuator applications it is important to know what the valve port flow conditions are as it shifts. As shown in these figures, dashed lined boxes show crossover condition. Normally discussions about crossover conditions cover “open” or “closed” types; in reality, the crossover condition may be a combination of these and may be different on either side of center. Open crossover stops shock while the spool shifts, while a closed crossover reduces actuator override travel. If the crossover condition is important to the circuit or machine function, show it on the schematic drawing.
Figure 8-47. Open crossover or transition condition.
Figure 8-48. Closed crossover or transition condition.
Figure 8-49 shows an all ports blocked center condition, solenoid pilot-operated valve, as a simplified and complete symbol. On most schematics, the simplified symbol is sufficient. The solenoid slash and energy triangle in the operator box show the valve has a solenoid operated valve piloting a pilot-operated valve. The boxes show the function of the main or working spool that controls the actuator. On valves with other hardware added (here, pilot chokes and stroke limiters), it is better to show the complete symbol. Both symbols in Figure 8-49 represent the same valve. The complete symbol gives more information about the valve function and helps with troubleshooting and valve replacement.
Figure 8-49. Solenoid pilot-operated valve with pilot chokes and stroke limiters. Internal pilot supply (X) and external drain (Y).
Using 5-way air valves
The 5-way selector valve and shuttle valve in Figure 8-50 works where a 3-way selector may not. The 3-way selector does fine when going from low to high pressure, but if there is no air usage to allow expansion, it is almost impossible to go from high to low pressure.
Figure 8-50. Pressure selector.
The 5-way and shuttle valve arrangement gives an exhaust path for high-pressure air when shifting to low pressure. After the air exhausts to the lower pressure, PR.1, the shuttle shifts and low pressure holds in the system.
Figure 8-51 shows a pair of 5-way valves piped to act like a three way light switch. Either valve moves the cylinder to its opposite position when activated.
Figure 8-51. Operating an actuator from two locations.
Figure 8-52 shows the normal hookup of a 5-way valve. Normally, input air goes to the center port of the side with three ports. A lot of air valve manufacturers call this #1 port. In the at rest condition, air flows from #1 to #4 port and on to the cylinder rod end, while #2 port exhausts the cylinder cap end through #3 port.
Figure 8-52. Operating a double-acting cylinder with one 5-way valve.
After shifting the valve, or extending, air flows from #1 port through #2 port to the cylinder cap end. Flow from the cylinder rod end goes to #4 port and exhausts through #5 port. The exhaust ports often have speed control mufflers to reduce noise and control the amount of exhaust flow. Speed control mufflers give individual meter-out speed control in each direction of travel.
Deenergizing the solenoid, or retracting, lets the valve spring return to its normal condition causing the cylinder to retract.
In Figure 8-53, the 5-way has a dual inlet instead of dual exhaust. Use a spool type valve for this hookup, since it takes pressure at any port without malfunction.
Figure 8-53. Air-saving circuit using a 5-way valve.
On most air circuits the cylinder does little or no work on the retract stroke. Putting low pressure on the rod side of the cylinder uses less compressor air without affecting the operation. This air savings results in lower operating cost and leaves more air to run other actuators. Install flow controls in the lines to the cylinder ports for individual speed control.
If the valve is solenoid pilot-operated, the supply to the pilot valve usually comes from port #1. This means, with a dual pressure inlet, pilot supply must come from some other source. On the circuit in Figure 8-53 a pilot line from system pressure goes directly to the pilot valve. System pressure goes into the external pilot supply port and a plug shuts off the internal pilot port. Changing the pilot line in the field with assistance from the supplier’s catalog is quite easy.
Figures 8-54 to 8-61 show another reason for using dual pressure inlets. They depict air cylinder movement with conventional hookup. The cylinder pauses before raising and drops rapidly when starting to retract.
Dual-pressure 5-way valves for air cylinder actuation
A vertical, up-acting air cylinder, with a heavy load, gives sluggish and jerky operation when valved conventionally. Figure 8-54 shows a conventional 5-way valve hook up on a cylinder raising a 600-lb load. This figure shows weight, cap and head end areas, and pressures at both cylinder ports.
Figure 8-54. Cylinder at rest.
When the directional valve shifts, as seen in Figure 8-55, there is a pause before the cylinder extends. The weight-to-cylinder force ratio and the rate of cylinder travel speed control the length of pause. The heavier the weight and the slower the cylinder speed, the longer the pause. The delay could be three to four seconds in extreme cases.
Figure 8-55. Valve just shifted, cylinder pauses.
The pause comes from weight pushing down along with force from air pressure on the cylinder rod end. At the moment the valve shifts to extend the cylinder, down forces are up to 1240-lb while up force is only 800 lb. As long as down forces exceed up force, the cylinder will not move. The slower the air exhausts, the longer it takes to get enough differential pressure across the cylinder piston to move it. The speed of exhausting air controls how fast the cylinder moves once it starts.
When pressure in the head end of the cylinder reaches about 15 psi, as shown in Figure 8-56, the cylinder starts to move. It moves up smoothly and steadily as long as the load remains constant.
Figure 8-56. Cylinder starts to move after rod-end pressure drops.
When the valve shifts to retract the fully extended cylinder, there is another problem. Figure 8-57 shows the cylinder at rest at the top. Up force is 800 lb from air pressure on the cap end, and down force is 600 lb from the weight.
Figure 8-57. Cylinder travels to end of stroke.
When the directional valve returns to normal, as shown in Figure 8-58, down force quickly changes to 1240 lb. Now the load drops rapidly until air pressure in the cap compresses to approximately 120 psi. It takes about 120 psi on the 10-in.2 area to slow the cylinder’s rapid retraction.
Figure 8-58. Valve shifted to retract cylinder, which drops rapidly.
Both pauses that occur when extending and retracting are eliminated by using the dual-inlet feature of a 5-way valve.
With a dual inlet pressure circuit shown in Figure 8-59, the cap end port has 80 psi while the rod end port is only 15 psi. This sets a pressure differential across the piston before the valve shifts.
Figure 8-59. Dual-pressure valve at rest.
When the valve shifts, as seen in Figure 8-60, down force is 720 lb and up force is 800 lb. The cylinder starts to move almost immediately and continues moving smoothly to the end.
Figure 8-60. Valve shifts, cylinder starts moving quickly.
In Figure 8-61 the valve shifts and the cylinder retracts. With the head end regulator set at 15 psi, down force from air pressure and the load is almost offset by up force. The load lowers smoothly and safely without lunging or bouncing, as fast as cap end air exhausts. In figure 8-59 to 8-61, the cylinder strokes smoothly and quickly in both directions with dual-pressure valve.
Figure 8-61. Valve shifts to normal, cylinder moves with no lunge.
Check valves as directional valves
Normally a check valve is not thought of as a directional control valve, but it does stop flow in one direction and allow flow in the opposite direction. These are two of the three actions a directional control valve can perform. An inline check valve stops any chance of reverse flow and is useful and/or necessary in many applications. Figure 8-62 shows the symbol for a plain check valve.
Figure 8-62. Plain check valve.
Another application for a check valve is a relief function, which can be seen in Figure 8-63. Heat exchangers, filters, and low-pressure transfer pumps often need a low-pressure bypass or relief valve. A check valve with a 25-125 psi spring makes an inexpensive, non-adjustable, flow path for excess fluid. It protects low-pressure devices in case of through flow blockage. Pilot operated directional valves commonly use a check valve in the tank or pump line to maintain at least 50-75 psi pilot pressure during pump unload. Some manufacturers make a check valve with an adjustable spring, for pressures up to 200 psi or more.
Figure 8-63. Backpressure check valve
Some check valves have a removable threaded plug in them that may be drilled to allow controlled flow in the reverse direction. The symbol in Figure 8-64 shows how to represent this in a symbol. A common use for a drilled check valve is as a fixed, tamper proof, flow control valve. Fluid free flows in one direction, but has controlled flow in the opposite direction. The only way to change flow is to change the orifice size. This flow control valve is not pressure compensated.
Figure 8-64. Check valve with orifice plug.
Many of the circuits in this manual show standard check valves in use. Hi-L pump circuits, reverse free flow bypass for flow controls, sequence valves or counterbalance valves, and multi-pump isolation, to name a few. Figure 8-65 shows some other applications for check valves.
Figure 8-65. Check valves in different circuit applications.
When the tank is higher than the pump or directional valves, always install some means to block flow lines for maintenance. If the valves are not blocked, the tank must be drained when changing a hydraulic component. Shut-off valves are the only option for lines that flow out of the tank to a pump or other fluid using device. To avoid running the pump dry, its shutoff should have a limit switch indicating full open before the electrical control circuit will allow the pump to start. All return lines though, can have a check valve piped as shown in Figure 8-65. A check valve with a low-pressure spring, called an tank isolation check valve, on each return line allows free flow to tank, while blocking flow out of it. A check valve in the tank lines makes shut off automatic and eliminates chances of blowing a filter or wrecking a valve at startup.
The backpressure check valve in the pump line maintains a minimum pilot pressure while the pump unloads. Here it is in the line feeding the directional valves, other times it is in the tank line. In either case it provides pilot pressure to shift the directional valves when a new cycle starts.
The circuit in Figure 8-65 also shows an anti-cavitation check valve for the cylinder with a relief valve to protect it from over pressure. An external force can pull against the trapped oil in the cylinder and cause damage or failure without relief protection. When outside forces move the cylinder, fluid from the rod end goes to the cap end, but is not enough to fill it. If a void in the cap of the cylinder is no problem then an anti-cavitation check valve is unnecessary. However, this void can cause erratic action when the cylinder cycles again, so install an anti-cavitation check valve. The anti-cavitation check valve has a very low-pressure spring, which requires 1-3 psi to open, so it allows tank oil to fill any vacuum void that might form. The anti-cavitation check valve has no effect during any other part of the cycle.
Pilot-operated check valves
There are some circuits that need the positive shut off of a check valve but in which reverse flow is also necessary. The following images show symbols of pilot-operated check valves that allow reverse flow. Figure 8-66 shows the symbol for a standard pilot to open check valve. Figure 8-67 shows a pilot-operated check with a decompression feature. The symbol in Figure 8-68 shows a pilot-operated check valve with an external drain for the pilot piston. Each of these pilot-operated check valves allow reverse flow, but two of them have added features to overcome certain circuit conditions.
Figure 8-66. Pilot-operated check valve.
Figure 8-67. Pilot-operated check valve with decompression poppet.
Figure 8-68. Pilot-operated check valve with external drain.
To hold a cylinder stationary, it must have resilient continuous non-leaking seals, no plumbing leaks, and a non-leaking valve. Metal-to-metal fit spool valves will not hold a cylinder for any length of time. As shown in Figure 8-69, a blocked center valve can actually cause a cylinder to creep forward. Vertically mounted cylinders with down acting loads always creep when using a metal-to-metal fit spool valve. Hydraulic motors always have internal leakage so the circuits shown here will not hold them stationary. Figures 8-70, 8-71, and 8-72 show a typical pilot-operated check valve circuit that prevents cylinder creep.
Figure 8-69. Blocked center directional valve, cylinder creeping forward.
The circuit in Figure 8-70 shows a horizontally mounted, non-leaking cylinder, positively locked in place any time the directional centers. When using an on-off type solenoid valve, a fast moving cylinder stops abruptly when the directional valve centers. Use a proportional valve with ramp timers to decelerate the actuator and eliminate shock damage.
Figure 8-70. Pilot-operated check circuit at rest with pump running.
Notice the directional valve has A and B ports open to tank in the center condition. This center condition allows pilot pressure to drop and the pilot-operated check valves to close. Using a directional valve with blocked A and B ports in center condition, may keep the pilot-operated check valves open and allow cylinder creep. If it is only necessary to keep the cylinder from moving in one direction, one pilot-operated check valve will suffice.
When solenoid A1 on the directional valve shifts, as seen in Figure 8-71, the cylinder extends. Pump flow to the cylinder cap end builds pressure in the pilot line to the rod end of the pilot-operated check valve, causing it to fully open. The pilot-operated check valve in the line to the cap end opens by pump flow like any check valve. Energizing and holding a directional valve solenoid causes the cylinder to move. Pilot operated check valves positively lock the cylinder but are invisible to the electric control circuit.
Figure 8-71. Pilot-operated check circuit with cylinder extending.
When solenoid B on the directional valve shifts, as seen in Figure 8-72, the cylinder retracts. Pump flow to the cylinder rod end builds pressure in the pilot line to the cap end of the pilot-operated check valve, causing it to fully open. The pilot-operated check valve in the line to the rod end opens by pump flow like any check valve. Energizing and holding a directional valve solenoid causes the cylinder to move.
Figure 8-72. Pilot-operated check circuit with cylinder retracting.
The following will describe how pilot-operated check valves can cause problems in some applications.
Pilot-operated check valves
Figure 8-73 shows how using a pilot-operated check valve to keep a heavy platen from drifting can cause problems.
| Figure 8-73. Pilot-operated check valve on running away load, at rest, pump running. |
When a cylinder has a load, trying to extend it causes load-induced pressure. In the example cited, a 15,000-lb platen pulling against a 26.51 square inch rod end area gives a 566 psi load-induced pressure. This load-induced pressure holds against the poppet in the pilot-operated check valve, forcing it closed. The pilot piston must have sufficient pressure to open the poppet with 566 psi pushing against it. The pilot piston on most pilot-operated check valves has an area that is three to four times that of the poppet. This means it will take approximately 141-188 psi at the cap end cylinder port to open the poppet for reverse flow.
When the directional valve shifts, starting the cylinder forward, as shown in Figure 8-74, pressure in the cap end cylinder port starts climbing to 150 psi. At about 150 psi the poppet in the pilot-operated check valve opens and allows oil from the cylinder rod end a free flow path to tank. The cylinder immediately runs away, pressure in cylinder cap port drops, the pilot-operated check valve closes fast and hard, and the cylinder stops abruptly. When the pilot-operated check valve closes, pressure at the cap end cylinder port again builds to 150 psi, opening the check valve, and the process starts again. A cylinder with these conditions falls and stops all the way to the work unless it meets enough resistance to keep it from running away.
Figure 8-74. Pilot-operated check valve on running away load, cylinder extending, free fall.
With this circuit, system shock very quickly damages piping, cylinders, and valves.
Adding a flow control between the cylinder and pilot-operated check valve is one way to keep it from running away. However, the restriction could cause fluid heating and slow cycling, and would need frequent adjustment to maintain optimum control.
Placing a flow control after the pilot-operated check valve causes backpressure against its pilot piston and could keep it from opening at all. With the flow control after the pilot-operated check valve, use one with an external drain. When there is much backpressure on the outlet of a pilot-operated check valve, it is best to use one with an external drain.
It is best to control the cylinder shown here with a counterbalance valve. See chapter five for the different types of counterbalance circuits.
Even with some spool type counterbalance valves, the cylinder still drifts. Adding an externally drained pilot-operated check valve between the counterbalance valve and the cylinder holds it stationary. The counterbalance valve keeps the cylinder from running away no matter the flow variations, while the pilot-operated check valve holds it stationary when stopped.
Figure 8-75. Pilot-operated check valve on running away load, cylinder stopping on closed P.O. check.
A pilot-operated check valve with the decompression feature would not help in this circuit.
Figures 8-76 and 8-78 show another possible problem using a pilot-operated check valve to keep a vertical down-acting cylinder from drifting. The cylinder in this example has a heavy weight pulling against the rod side. A load induced pressure of 1508 psi plus 142 psi from pilot pressure acts against the poppet in the pilot-operated check valve. This requires a high pilot pressure to open the pilot-operated check valve.
Figure 8-76. Pilot-operated check valve on running away load, cylinder just starting to extend.
It requires approximately 500 psi pilot pressure to open the pilot-operated check valve with 1650 psi against the poppet. As pilot pressure builds to open the poppet, it also pushes against the full piston area of the cylinder. This cylinder has nearly twice the area on the cap side as the rod side, so every 100 psi on the cap side gives about 200 psi on the rod side. As pilot pressure builds to the 500 psi required, pressure against the poppet in the pilot-operated check valve increases at twice the rate. Figure 8-77 shows the start of this condition.
Figure 8-77. Pilot-operated check valve on running away load, cylinder still trying to extend.
In Figure 8-77, the cylinder rod end pressure is at 300 psi, which adds 570 psi to the 1508 psi load-induced pressure. The extra hydraulic pressure pushes harder against the pilot-operated check valve poppet, making pilot pressure increase even more.
As pilot pressure increases, down force and rod end pressure escalates also. In Figure 8-78, rod end pressure is at 3565 psi because pilot pressure continues to climb. In the situation shown here, it is obvious the relief valve will open before reaching a pilot pressure high enough to open the pilot-operated check valve. Even if pilot pressure could go high enough to open the pilot-operated check valve, the cylinder runs away and stops.
Figure 8-78. Pilot-operated check valve on running away load, cylinder still trying to extend.
A pilot-operated check valve with a decompression poppet would not help in this situation. Flow from the small decompression poppet is not enough to handle cylinder flow. The cylinder would extend with a decompression poppet, but at a very slow rate.
It is best to control the cylinder in this example with a counterbalance valve. See chapter five for the different types of counterbalance circuits.
Even with some spool type counterbalance valves, the cylinder still drifts. Adding an externally drained pilot-operated check valve between the counterbalance valve and the cylinder will hold it stationary. The counterbalance valve keeps the cylinder from running away no matter the flow variations, while the pilot-operated check valve holds it stationary when stopped.
Shown are circuits that require a pilot-operated check valve to have external drain and/or decompression capabilities.
A standard pilot-operated check valve circuit usually has minimum backpressure at the reverse flow outlet port. If there is a restriction causing high backpressure in the reverse flow outlet port, a standard valve may not open when applying pilot pressure. The reason this might happen is the pilot piston sees backpressure from the reverse flow outlet port. If the pilot-operated check valve poppet has load induced pressure holding it shut, plus reverse flow outlet port backpressure opposing the pilot piston, there is not enough pilot piston force to open the check poppet.
If the reverse flow outlet port backpressure cannot be eliminated, then specify a pilot-operated check valve with an external drain. Pipe the external drain to a low or no pressure line going to tank. With an external drain pilot-operated check valve, the pilot piston usually opens the check poppet to allow reverse flow.
Figure 8-79. Pilot-operated check valve circuit with external drain function at rest, pump running.
The schematic drawing in Figure 8-79 shows a cylinder with pilot-operated check valves at each port and meter out flow controls downstream of the reverse flow outlet port. If this circuit did not have externally drained pilot-operated check valves, the cylinder would operate in jerks or not at all when the directional valve shifts. Backpressure from the flow controls can push the pilot piston closed and stop the cylinder, then pressure would drop and it would start again. This oscillating movement would continue until the cylinder competes its stroke. With externally drained pilot-operated check valves, the cylinder is easy to control at any speed.
Placing the flow controls in Figure 8-79 between the cylinder ports and the pilot-operated check valve eliminates backpressure. This move eliminates the need for externally drained pilot-operated check valves.
In Figure 8-80, a running away load had a drifting problem with only the counterbalance valve installed. Adding a pilot-operated check valve in front of the counterbalance valve stopped cylinder drifting. Using a decompression poppet made it easy to open the main check poppet against the high load induced pressure. The decompression poppet releases trapped fluid in the piping between the pilot-operated check valve and the counterbalance valve allowing the main check poppet to open.
Figure 8-80. Pilot-operated check on running away load with external drain and decompression poppet with P.O. check for no leak holding, counterbalance valve for smooth control of the extend stroke at rest, with pump running.
Notice the pipe between the pilot-operated check valve and the counterbalance valve is at zero psi while the cylinder is held retracted. This pressure would have been about 1200 psi while the cylinder was retracting, but quickly drops to zero when the directional valve centers. The reason for this pressure drop is leakage past the counterbalance valve spool, which is the reason for adding the pilot-operated check valve.
If the pilot-operated check valve did not have an external drain, backpressure from the counterbalance valve can force it shut when the cylinder starts moving. The external drain and decompression features are both necessary in this holding circuit.
Placing the pilot-operated check valve in the line after the counterbalance valve would require neither an external drain nor decompression feature. However, the reason for installing the pilot-operated check valve was to stop drifting. With the pilot-operated check valve after the counterbalance valve, the counterbalance valve must have an external drain. An external drain indicates there is internal leakage, so the drift problem may decrease -- but would not go away.